This invention relates to a ring laser gyros, and more particularly to an actuator for controlling laser light path length in a ring laser gyro by changing the position of a mirror.
Prior art path length actuators for ring laser gyros are described in U.S. Pat. No. 4,160,184 ("Piezoelectric Actuator") issued to Bo Ljung. Such actuators generally comprise a plurality of piezoelectric discs assembled into a stack. A stack can implement the required stroke if it is made sufficiently large.
The main disadvantage with prior art designs is their complexity; the designs involve many piezoelectric elements and many adhesively bonded interfaces. This complexity affects the cost of the actuator and the reliability. Moreover, it is difficult if not impossible to optimize the piezoelectric stack to a particular spring constant of the mirror assembly. This causes the designer to choose a massive stack which can provide sufficient force to move the path length mirror the required distance.
Further disadvantages arise in bonding a stack of piezoelectric discs with adhesive. Maintaining electrical contact to an outside source requires the use of conductive adhesives, which unfortunately are poor in quality an can lose their conductive properties over time. In addition, because the thermal coefficient of expansion for piezoelectric materials changes as a function of age, a match to other low expansion materials used in the ring laser gyro construction is difficult or impossible.
A stack of piezoelectric wafers is usually rigid not only axially but also rotationally. If the stack is imperfectly bonded, it will rotate by tilting when energized. It is common that such stack tilt will couple into the mirror. Such a rotation, if it is larger than 0.01 seconds of arc, is deliterious for the ring laser gyro. Such tilts cause a displacement of the laser beams inside the gyro, which under certain conditions can cause a bias shift.
An object of the present invention is to provide a structurally simple, inexpensive and reliable path length actuator wherein the transmission of torques from the actuator to the mirror is minimized if not eliminated. Another object of the present invention is to provide such an actuator in which the circuit energizing the piezoelectric elements has a maximum conducting life and in which the structural mountings of the piezoelectric elements maintain a high level of strength. Yet further objects of the present invention are to provide such an actuator in which the matching of the spring constant of the piezoelectric assembly to the spring constant of the mirror assembly is facilitated and in which differential thermal expansion is compensated.